Methods for producing a granular metal oxide material by a sol-gel process are commonly known. For example, silica flakes are produced by: 1) hydrolyzing and condensation-polymerizing a silicon alkoxide in an aqueous alcohol solution to form a silica sol; 2) applying the silica sol onto a substrate to form a thin film; and 3) peeling the thin film from the substrate. The use of an alkaline silica sol results in embrittlement of the thin film, leading to reduction in production yield. Therefore, an acidic silica sol is suitable for the above method.
The above silica flakes are called glass flakes, and used by being dispersed in a matrix formed of another material. For example, a resin molded body can be improved in its strength and dimensional accuracy by addition of glass flakes. There are also known bright pigments whose reflectivity has been increased by covering the surfaces of glass flakes with a film of metal or metal oxide. Bright pigments are blended with cosmetics, inks, etc., to enhance their commercial values.
The details of methods for producing glass flakes using a sol-gel process are disclosed, for example, in Patent Literature 1 to 4.
There has also been proposed a method including: hydrolyzing a metal alkoxide at an interface between two liquid phases, a phase of an aqueous medium and a phase of water-insoluble medium phase, thereby forming a ceramic thin film precursor; and sintering the obtained ceramic thin film precursor to obtain a ceramic thin film (see Patent Literature 5).
There has also been proposed a method including: partially hydrolyzing a chemically-modified metal alkoxide to form a polymer; dissolving the polymer in a solvent that is soluble in water to obtain a solution; spreading the solution on a water surface to form a gel nanosheet; drying and sintering the gel nanosheet to obtain an oxide ceramic nanosheet (see Patent Literature 6).
Granular silica materials account for the largest proportion of granular metal oxide materials actually mass-produced by sol-gel processes. However, sol-gel processes can be carried out also by using a metal alkoxide containing another metal element such as titanium or zirconium. The ultraviolet-shielding function and photocatalytic function of fine particles of titanium oxide obtained by sol-gel processes have been attracting attention.